It is desirable in many situations to have porous materials that will repel water and oil, but at the same time allow the passage of air and other gases. This is particularly true in the case of microporous materials which are designed in many instances to allow the passage of a particular gas while preventing the passage of a broad spectrum of liquids. This repellency characteristic is often achieved by treating substrates such as paper, fabric or polymers, which have a porous structure with some type of chemical that will render them oil and/or water repellent. Ideally, the treatment should not close the pores of the substrate or otherwise restrict the flow of gases therethrough.
Repellency treatments are well known; however, prior art techniques often suffer from the disadvantage that repeated exposure to water or oil, such as by washing or dry cleaning, reduces the effectiveness of the treatment and the substrate eventually loses it water and oil repellency. As a result, it is frequently necessary to retreat substrates after washing or dry cleaning in order to maintain the desired repellency characteristics.
Use of fluorine containing polymers to provide water and oil repellency to textiles has been practiced for many years. Scotchguard.RTM. sold by the 3-M Company and Zonyl.RTM. sold by DuPont are examples of such treatments. The effectiveness of this treatment is reduced by dry cleaning with solvents such as Perchloroethylene or Freon.RTM. and reapplication of the treatment is required after dry cleaning.
In some applications, methods and products have been developed for treating substrates such that the water and oil repellency characteristics of the substrate are allegedly maintained even after repeated washings or exposure to water and oil. For example, European patent application publication No. 0 193 370 A2 describes a particular group of fluorinated polyacrylates and polyacrylamides having a controlled degree of cross-linking and methods for preparing them. This publication discloses the use of a monomer of monoacrylate or monoacrylamide and a monomer of diacrylate or diacrylamide to prepare fluorinated polymers (column 1, lines 56-59; column 2, lines 1-10). Polymerization and cross-linking of the monomers is achieved by the application of ultraviolet radiation or electron beam radiation. (column 6, lines 40-43). The publication teaches that the treatment can be used on any substrate to provide water and oil repellency. (column 2, lines 61-65; column 3, lines 1-5; column 4, lines 3-5; column 5, lines 44-50). The application does not discuss the treatment in the context of a porous substrate in which the polymerization and cross-linking is achieved in situ on the surface of the substrate in a single step so as to form a conformal, mechanically adhered coating upon the substrate. Also, it should be noted that the particular materials described and claimed herein are not shown in the European application.
Japanese published application 60.39482 discloses a process for rendering textiles soil repellant through the use of a copolymer of a fluorinated and a non-fluorinated monomer. This application teaches that the use of a series of fluorinated monomers (which differ from those of the present invention) is responsible for making the textile oil repellent; and also, it emphasizes the necessity of using the non-fluorinated monomer to provide washability. The present invention employs different fluorinated monomers and does not require the use of the non-fluorinated monomer, yet provides greatly enhanced washing resistance. Yet another approach is disclosed in U.S. Pat. No. 3,847,657 which teaches a process for physically grafting a fluorinated monomer onto a polyester fiber through a free radical initiated process. This method, and the products thereof, are fundamentally different from those of the present invention.
Two types of wettability phenomena are of interest concerning the water and oil repellency of porous substrates. The first phenomenon relates to the tendency of the substrate to resist the transfer of liquid therethrough. This relationship is described by the following mathematical formula: ##EQU1## in which P is the breakthrough pressure required to force liquid through the substrate; gamma (.gamma.) is the surface tension of the liquid; theta (.theta.) is the contact angle formed between the liquid and a smooth surface of the solid material; and D is the effective pore diameter of the substrate. The contact angle (.theta.) is dependent on the particular liquid and solid involved, and the surface tension (.gamma.) is a characteristic of the liquid.
The purpose of treating the substrate is to increase its repellency by increasing the contact angle above 90.degree.. According to the formula, a positive value for the applied pressure is possible only for negative values of the cos .theta.. Thus only when the contact angle is greater than 90.degree. is a positive breakthrough pressure required to force the liquid through the substrate. It should be appreciated that by treating the substrate, the contact angle will change because it is a unique function of a particular liquid that a particular surface, and the surface is changed by the treatment. In a hydrophobic porous substrate, the contact angle for water is greater than 90.degree. and one has to apply a liquid pressure in order to overcome the resistance of the substrate to the transfer of water therethrough. At a contact angle less than 90.degree. the substrate is intrinsically wetable and liquid will normally pass through in the absence of an externally applied force.
As mentioned above, the wetting or contact angle is also a function of the liquid involved. There is an empirical, almost linear relationship between cos .theta. and the surface tension of the liquid. The smaller the surface tension, the larger the cos .theta. becomes. (Reference: S. Wu in "Polymer Interface and Adhesion," Marcel Dekker 1982, p. 183). Most organic liquids have much lower surface tension (range of 18-40 dyne/cm) than that for water (72 dyne/cm) and are therefore more capable of wetting and penetrating porous substrates. The efficiency of an oil repellent treatment is therefore normally characterized by the lowest surface tension fluid which does not wet and penetrate the substrate. Solvents such as hexane (gamma=18 dyne/cm) are amongst the lowest surface tension fluids and the most challenging for those substrates.
It should be appreciated that the breakthrough pressure (P) is also inversely proportional to the pore diameter (D). Thus, microporous repellent substrates with pore diameters of 0.1 to 1 micron such as membranes and the like have breakthrough pressures ten or more times greater than substrates with pore sizes of 10 microns or more such as fabrics.
The second parameter relating to the repellency of a porous substrate involves the friction of the liquid as it moves across the substrate's surface. This parameter can be of critical importance especially in the case of a microporous membrane; because, even if a liquid does not pass through the membrane it can attach and coat the surface of the membrane such that air or gas permeability through the membrane is minimized. Ideally, a substrate should retain no liquid on its surface after exposure to the liquid.
The frictional force acting on a liquid to prevent movement across a substrate can be measured by a sliding drop test. In this test, a constant size drop (i.e., 50 ul or 25 ul) is placed on a substrate while it is in a horizontal position. The substrate is then tilted to the angle at which the drop first starts to move. This angle is called the sliding angle, and the smaller the sliding angle the greater the tendency of the liquid to drain from a surface after it is exposed to liquids. The sliding angle depends on the size of the drop. It becomes smaller as the drop becomes heavier. Surface morphology is also very important. Experience shows that the sliding angle becomes smaller with more open surface, provided the surface is hydrophobic enough. This was shown to be true with, for instance, microporous PTFE membranes. The higher nominal pore size PTFE membrane showed lower sliding angles. The same is also true with the treatment of this invention.